This invention relates to air distribution systems and more particularly to apparatus for extracting air from a main supply duct to a branch duct.
In a conventional air distribution system in a building, the air is branched from the main air supply duct to the various branch ducts through openings in the wall of the main duct which enter into the branch ducts.
Generally, the volume flow rate through the branch is determined by the static pressure in the main duct and the flow resistance of the branch. Since the branch opening is flush with the wall of the main duct, the dynamic pressure of the flow in the main duct does not contribute to the flow rate in the branch.
In such configurations, the noise level at the entrance to the branch duct is substantially the same as the noise level in the main duct. This level is generated mainly by the air supply fan, which noise travels through the main air duct without much attenuation. In the prior art, to reduce the noise level, a silencer is typically used at the exit of the fan in the main supply duct. Frequently a silencer is also incorporated at the inlet to the main supply fan.
In order to minimize the effects of the silencer on the system, the silencer must have a low pressure drop and its total open area must be large. Thus, if adequate acoustic attenuation is to be achieved, the silencer dimensions must be made quite large. This means that the silencer has the disadvantage, not only of being costly, but also bulky, requiring a large amount of space. If the dimensions of the silencer are reduced, the pressure drop will increase and it may then be necessary to select a larger fan to achieve the required total volume flow rate through the main duct. This latter alternative is extremely costly from an energy standpoint.
In order to further attack the noise problem, silencers may be introduced in the branch ducts, or alternatively, the branch ducts may incorporate noise attenuating liners. If should be kept in mind, however, that in order for such a silencer or liner to be effective at low frequencies, the absorptive elements must be quite thick, and in order for the pressure drop in the branches to be kept to an appropriately low value, the dimensions must be correspondingly large. This leads to impractical distribution systems.
It is an object of the present invention to provide a composite branch duct take-off and silencer.
It is a further object to provide each branch duct with noise attenuation but in such a manner that the dimensions of the ducts remain unchanged.
It is another object to provide a composite branch take-off and silencer, eliminating the need for a silencer at the fan.
Yet another object is to provide a composite branch take-off and silencer providing relatively high air handling capacity and the volume flow in a branch duct.